Liquid wastes discharged in various photographic processing steps are collected generally in a mixed state, and treated as photographic waste liquid. Diversity of the waste liquid composition makes it difficult to find effective methods for waste liquid treatment. So the photographic waste liquid is one of the most difficult-to-treat industrial effluents. Although a number of treatment methods for photographic waste liquid have so far been disclosed, they still have many problems with both removal rate and treatment cost.
As a realistic countermeasure, photographic waste liquid is consigned to persons involved in recovery and disposal of effluent, and incinerated by them. For performing incineration without emitting ecologically deleterious substances into the air and water environments, it is necessary to raise the incineration temperature. However, incineration disposal is difficult to perform by continuous operation of a middle- or small-scale incinerator at high temperatures. So the incineration has to be carried out using a large-scale incinerator and cannot help involving a high disposal cost. In addition, for the purpose of avoiding pipe clogging and wearing away of the incinerator by high melting inorganic salts, such as iron oxides, produced by burning, it is required to provide a chemical iron-removal step. Therefore, the disposal by incinerator has a problem with further complexity of disposal process and operation.
Under these circumstances, the incineration disposal, though a realistic countermeasure, is not a satisfactory countermeasure at present. So studies of more excellent treatment techniques for photographic waste liquid have been made continually.
The methods hitherto disclosed with respect to the treatments for photographic waste liquid are mainly biological, chemical and physical treatments.
As examples of the biological treatment are disclosed many methods of treating photographic waste liquid by use of activated sludge, inclusive of the method for reducing COD of waste liquid from medical X-ray film processing as disclosed in JP-A-59-42094. In these treatment methods, waste liquid diluted generally by a factor of 10 to 50 is treated with activated sludge for a period (an average residence time) of 15 to 50 days. As a result, the waste liquid is said to receive a 50-80% reduction of COD and a 50-80% reduction of BOD by decomposition and removal.
Examples of chemical treatment (oxidation method) include an ozone oxidation method, a hydrogen peroxide oxidation method, oxidation methods using other chemical oxidants, and an electrolytic oxidation method. The ozone oxidation method is disclosed, e.g., in JP-A-7-47347. Although it is an effective method for decomposition and removal of inorganic ingredients as a COD component and decomposition of benzene rings contained in aromatic developing agents, ozone oxidation has a minimal effect on the removal of organic ingredients as a BOD component. As to the method of using hydrogen peroxide, the method of using hydrogen peroxide in combination with a catalyst is disclosed in JP-A-9-234475. In addition, the hydrogen peroxide-ferric salt method (Fenton method), though effective for both inorganic and organic ingredients, is attended with a treatment-cost problem. Further, there are known the method of using persulfate as an oxidant, the method of adding an oxidant to a strong acidic solution and thereby depositing sulfur compounds in a stabilized condition, the method of oxidizing with chlorine and hypochlorite, and the treatment method of heating in the presence of persulfate.
Of those methods, the electrolytic oxidation method has advantages that it ensures easy and safe operations and enables reduction of apparatus size in contrast to oxidation treatment with a strong chemical oxidizing agent, and simple in comparison with biological treatment and physical treatment. The electrolytic oxidation methods are disclosed, e.g., in JP-A-63-116796, JP-A-8-296081 and JP-A-7-323290. In the process of electrolyzing photographic processing waste liquid, however, oxidized species produced upon electrolysis decompose organic substances with efficiency when the organic component concentration is high, but it occurs in many cases that at the time when the organic substances are decomposed into lower fatty acids including acetic acid, formic acid and oxalic acid the efficiency of further decomposition by electrolysis is lowered to result in waste of electric power.
In general, it is said that the COD component removal rate (reduction rate) attained by chemical treatment is of the order of 50%.
Examples of physical treatment include a high-pressure heating method, an atomization burning method and an evaporative drying method. Since a large amount of halide ions are contained in photographic waste liquid, stress corrosion of a reactor becomes a problem. In addition, the heat exchanger used for heat recovery has a problem with disposal of scales, residues and exhaust gases.
Further examples of hitherto proposed physical treatment include an adsorptive removal method using an inorganic or organic high polymer adsorbent, a reverse osmosis method and a dialysis method.
When any of the foregoing methods is used anone, however, it cannot have satisfactory treatment effect upon photographic waste liquid in which a wide variety of chemical substances causing environmental pollution are present. For instance, an problem with the chemical oxidation method (1) is a great cost increase caused by consumption of a large amount of chemical reagents, an problem with the electrolytic oxidation method (2) is a drop in COD reduction rate resulting from fouling of electrodes, an problem with the absorptive removal method (3) is an adsorbent usage increased with decreasing adsorption power, an problem with the evaporation method (4) is an release of an offensive order and toxic substances, an problem with the microbial treatment method (5) is a decrease of microorganisms' ability to treat COD components in the presence of deleterious substances, and an problem with the reverse-osmosis or dialysis method (6) is a short life of column or film.
As improvement measures, combinations of the foregoing methods, especially a combination of oxidation treatment and microbial treatment, have been proposed. For instance, JP-A-3-262594 discloses that both of COD and BOD of photographic waste liquid can be reduced by the combination of hydrogen peroxide oxidation treatment (Fenton method) and microbial treatment, JP-A-4-235786, JP-A-6-320184 and JP-A-4-244299 disclose the electrolytic treatment-microbial treatment combinations enabling reductions in COD and BOD of photographic waste liquid, and JP-A-5-96298 discloses that the COD and BOD of photographic waste liquid can be reduced by using photochemical oxidation with ozone gas in combination with microbial treatment. However, these combination methods each involve any of such problems that the size of apparatus becomes large and thereby a large space for installation of the apparatus is required, operations become complicated, special microorganisms are required for treatment, and dilution with a large quantity of water is required. Therefore, any of them cannot afford a satisfactory solution.
Of those treatments, the electrolysis method used for treatment of waste liquid features easy and safe operations and reduction in apparatus size, compared with the oxidation treatment using strong chemical oxidants. However, in the process of electrolyzing photographic processing waste liquid, oxidized species produced upon electrolysis decompose organic substances with efficiency when the organic component concentration is high, but it occurs in many cases that at the time when the organic substances are decomposed into lower fatty acids including acetic acid, formic acid and oxalic acid the efficiency of further decomposition by electrolysis is lowered to result in waste of electric power. In the electrolysis of those organic acids, the potential window is still narrow even when platinum and lead electrodes are used, so there is no improvement in electrolysis efficiency. In addition, the use of platinum and lead electrodes causes a trouble of eluting heavy metal ions.
JP-A-7-299467 discloses the electrolytic oxidation method using a positive electrode having a diamond-evaporated surface. Therein, it is shown that such an electrode enables an increase in impressed voltage to result in enhancement of the effect of decomposing organic substances. However, any of the methods mentioned above still falls short of meeting the effluent standards of Sewerage Law.
On the other hand, in view of the composition of photographic waste liquid, biological treatments with aerobic microorganisms have a common inevitable drawback of requiring a large volume of dilution water for their application to photographic waste liquid and consequent upsizing of apparatus. Therefore, these treatments are practicable in large-scale waste-treatment sites where photographic waste liquid is accumulated and treated intensively, preferably together with other liquid wastes, but it is disadvantageous to carry out them in photofinishing laboratories where photographic waste liquid generates because the volume of waste liquid to be treated is increased by water dilution to result in increases of equipment cost, installation space and operation cost. Although the foregoing complex treatment techniques involving biological treatment can raise reduction rates of COD and BOD, they have problems peculiarly their own as mentioned above and disadvantages associated with water dilution, so it is impractical to apply them in photofinishing laboratories.
In contrast, anaerobe treatments can often be applied effectively to liquid wastes having high salt concentrations and high COD. However, anaerobic treatment cannot be applied to photographic waste liquid because the waste liquid has a high sulfur compound content and produces hydrogen sulfide by reduction under an aerobic conditions, thereby making it impossible for microorganisms to survive.
In photo finishing laboratories, as mentioned above, it is difficult to perform not only biological treatment, irrespective of whether it is aerobic or anaerobic, but also complex treatment methods wherein biological treatment is combined with other treatments. So photographic waste liquid is transported from photofinishing laboratories to an outside centralized treatment site to which the treatment thereof is commissioned. Under these circumstances, a waste treatment method which makes it possible for both BOD and COD of photographic waste liquid to be reduced to no higher than drainage standard values and has practicability in photofinishing laboratories also is desired strongly.
As mentioned above, any of the hitherto disclosed waste liquid treatment methods, irrespective of whether they are used in isolation or in combination, falls short of perfectly solving the problems of the hour, and they are difficult to carry out in photofinishing laboratories in particular. Therefore, it is strongly desired to find a method of reducing BOD and COD of photographic waste liquid to a level lower than the effluent standards, especially a photographic waste liquid treatment method which can be carried out in photofinishing laboratories also.